The metallic nanoparticles (NPs) assembled at the liquid-liquid interface (LLI) are shown to have widespread applications ranging from sensors to tunable optical devices. Optical actuation of LLI is a promising way to realize simple, cost-effective and reconfigurable optical components. Herein, we report a thermoplasmonically actuated all-optical modulator based on a heptane/water interface incorporated with assembled Au NPs.Method: The working principle relies on the temperature rise at the LLI due to plasmonic heating and controllable dewetting of the top liquid layer triggered by the thermocapillary flow. The system comprises of heptane layer as the top layer, water as the bottom layer and 2D array of Au NPs (size 51 ± 5 nm) at the interface. The temperature gradient to drive flow arises from the plasmonic heating using a focused 532 nm laser beam at the NP array. Results: The excitation of assembled Au NP layer at the plasmonic wavelength causes a localized temperature rise of 3.2 ± 0.7 oC, at the irradiated zone and creates a thermocapillary flow across the interface. The flow thus generated results in the deformation and subsequent rupturing of the heptane layer, when the thickness of the layer is below the capillary length (0.5 ± 0.05 mm). To operate the platform as an optical modulator, a signal beam (655 nm) was aligned to pass through the heptane layer, parallel to the interface. When the trigger beam is off, the layer is intact and the signal beam gives a maximum output (ON state). Upon switching on the trigger beam, the heptane layer ruptures and blocks the incoming signal, resulting in a minimum output (OFF state). Conclusions: We developed a thermoplasmonically-actuated optical modulator based on a liquid-liquid interface decorated with self-assembled Au nanoparticles. The working principle relies on thermoplasmonically controlled reversible rupturing of the top layer, which modulates the transmittance of the signal beam passing through this layer. A temperature rise of only 3.2±0.7 °C at the interface is sufficient to realize modulation in optical transmission.